Field of the Invention
The present invention relates to a lens apparatus and more particularly to a lens apparatus adapted to drive a movable optical member by electrical control. The present invention also relates to an image pickup system having such a lens apparatus.
Description of the Related Art
In conventional lens apparatuses used in moving image shooting, the change in the field of view relative to the amount of shift in zooming operation varies depending on the zoom position. (Hereinafter, the rate of change of the field of view relative to the zoom shift amount will be referred to as the field-of-view change rate.) Therefore, even when the zoom lens is driven at a constant speed using an actuator or the like, the field-of-view change rate is not constant, resulting in unnatural images in some cases. For example, when an object moving along the optical axis at a constant speed is to be shot in a specific image pickup area for a specific length of time while keeping the size of the object image constant, if the field-of-view change rate is substantially constant, the operation that the photographer needs to do is to input a command for driving the zoom at a constant speed for the specific length of time through a controller or the like. However, if the field-of-view change rate is not constant it is not possible by the above-described method (namely inputting a command for driving the zoom at a constant speed for the specific length of time) to keep the size of the object image constant within the image pickup area during image shooting, resulting in an unnatural image in some cases. To avoid this, the photographer needs to operate the controller (or adjust the zoom speed) in accordance with the size of the object image in the image pickup area. Such operation is troublesome.
Japanese Patent Application Laid-Open No. H11-211962 discloses a technology that makes the relationship between the focal length of a zoom lens and the field-of-view change rate variable when the speed signal for the zoom lens is constant. A zoom speed signal is set based on the varied field-of-view change rate and the position of the zoom lens and output to a drive circuit to drive the zoom lens.
One method for keeping the field-of-view change rate constant when the zoom lens is driven at a constant speed is to vary (increase and/or decrease) the zoom drive speed depending on the zoom position to make the field-of-view change rate substantially constant. However, in the case where this method is employed, if the zoom is driven at high speed, it is required in some cases to achieve a speed higher than the highest drive speed (maximum speed) at which the zoom lens can be driven. Then, even if it is required to drive a speed higher than the maximum speed, the drive speed is necessarily kept at the maximum speed, leading to an elongated drive time. Moreover, a sudden change in the acceleration is caused to possibly make the image unnatural. Moreover, the field-of-view change rate is not made substantially constant as desired.